The optical properties of zinc oxide (ZnO) have been studied for potential use in semiconductor devices, in particular for photonic light emitting devices such as light emitting diodes (LEDs), laser diodes (LDs) and photonic detectors such as photodiodes. The energy band gap of ZnO is approximately 3.3 electron volts (eV) at room temperature, corresponding to a wavelength of approximately 376 nanometer (nm) for an emitted photon of this energy. Light emission has been demonstrated from ZnO LEDs using p-type and n-type materials to form a diode. ZnO has also been used to fabricate a UV photodetector and a field effect transistor (FET).
ZnO has several important properties that make it a promising semiconductor material for optoelectronic devices and applications. ZnO has a large exciton binding energy, 60 meV, compared with 26 meV for GaN and 20 meV for ZnSe. The large exciton binding energy for ZnO indicates promise for fabrication of ZnO-based devices that would possess bright coherent emission/detection capabilities at room and elevated temperatures. ZnO has a very high breakdown electric field, estimated to be about 2×106 V/cm (greater than two times the GaAs breakdown field), indicating thereby that high operation voltages could be applied to ZnO-based devices for high power and gain. ZnO also has a saturation velocity of 3.2×107 cm/sec at room temperature, which is larger than the values for gallium nitride (GaN), silicon carbide (SiC), or gallium arsenide (GaAs). Such a large saturation velocity indicates that ZnO-based devices would be better for high frequency applications than ones made with these other materials.
Still further, ZnO is exceptionally resistant to radiation damage by high energy radiation. Common phenomena in semiconductors caused by high-energy radiation are the creation of deep centers within the forbidden band as well as radiation-generated carriers. These effects significantly affect device sensitivity, response time, and read-out noise. Therefore, radiation hardness is very important as a device parameter for operation in harsh environments such as in space and within nuclear reactors.
From the perspective of material radiation hardness, ZnO is much better suited for space operation than other wide bandgap semiconductors. For example, ZnO is about 100 times more resistant than is GaN against damage by high-energy radiation from elections or protons.
ZnO also has a high melting temperature, near 2000° C., providing possibilities for high temperature treatments in post-growth processes such as annealing and baking during device fabrication, as well as for applications in high temperature environments.
Large-area ZnO single crystal wafers (up to 75 mm diameter) are commercially available. It is possible to grow homo-epitaxial ZnO-based devices that have low dislocation densities. Homo-epitaxial ZnO growth on ZnO substrates will alleviate many problems associated with hetero-epitaxial GaN growth on sapphire, such as stress and thermal expansion problems due to the lattice mismatch.
ZnO has a shallow acceptor level, 129 meV, compared with 215 meV for GaN. The low value for the acceptor level means that p-type dopants in ZnO are more easily activated and thereby help generate a higher hole concentration in ZnO than the corresponding hole concentration in GaN for the same dopant concentration in each material. ZnO based devices can be fabricated by a wet-chemical etch process. These properties make ZnO a most attractive material for development of near- to far-UV detectors, LEDs, LDs, FETs, and other optoelectronic devices.
It would be desirable to modify the films and structures of ZnO based light emitting semiconductor devices and method of preparing such films and structures, to achieve one or more of the following: to contain and confine electrical carriers in the active region for increasing the efficiency of carrier combination for producing light, to provide waveguides for light generated in the device for increasing the efficiency for light extraction from the device, to obtain desired spectral intensity versus wavelength of light extracted from the device by modifying the path by which light generated in the device is extracted from the device, to obtain desired spectral intensity versus wavelength of light extracted from the device by modifying the properties of material layers in the path by which light generated in the device is extracted from the device, to obtain desired spectral intensity versus wavelength of light extracted from the device by addition of material layers such as phosphors in the path by which light generated in the device is extracted from the device, to provide for emission of a multiplicity of wavelengths from the active layer region, and to obtain stimulated emission of radiation in order to provide for increased function, capability and performance of semiconductor devices.
It would also be desirable to modify the films and structures of ZnO based light emitting semiconductor devices and method of preparing same to reduce leakage currents to values lower than those for present ZnO based semiconductor devices, to improve electrical contact to films and substrates to make them electrically less resistive and more ohmic than those for present ZnO based semiconductor devices, to improve the passivation properties of films to provide protection of the device from undesirable impurity elements and chemicals to make their lifetime longer than those for present ZnO based semiconductor devices in order to provide for increased function, capability and performance of semiconductor devices.
A material with band gap energy larger than ZnO in a ZnO based semiconductor device may be used to form cladding layers and barrier layers for confining electrical carriers and photons in a semiconductor device.
A material with band gap energy larger than ZnO in a ZnO based semiconductor device may be used to form blocking layers to alter the movement of electrical carriers in a semiconductor device. Blocking layers may be used as a blocking layer to reduce transport of electrons, ions, atoms and molecules. A blocking layer may be used to reduce flow of undesirable impurity species and thereby provide passivation resistance to a semiconductor device. Undesirable impurity species include H2, H, OH, CO2, H2O, O2, and chlorine. It would be desirable to form a passivation layer in a semiconductor structure that serves as a barrier layer to undesirable impurity species in order to provide for increased function, capability and performance of semiconductor devices. It would be desirable to form a semiconductor alloy layer with higher passivation resistance than the passivation resistance of ZnO in order to provide for increased function, capability and performance of semiconductor devices.
Wet chemical etching is a useful process in the preparation of ZnO based semiconductor devices. A material that is a semiconductor alloy of ZnO may be used to form a layer that has a lower etch rate than the etch rate of ZnO. The differences in etch rate may be used to form useful structures in ZnO based semiconductor devices. It would be desirable to form a semiconductor layer with a wet-chemical etch rate that is lower than the wet-chemical etch rate of ZnO in order to provide for increased function, capability and performance of semiconductor devices.
A layer in a semiconductor device that has an energy band gap that is larger than the energy band gap of a nearby layer can serve as a waveguide layer for light in the device.
A layer in a semiconductor device that has an energy band gap that is larger than the energy band gap of an adjacent layer can serve as a barrier layer to confine electrical carriers in a region of a ZnO based semiconductor device to improve the efficiency of the device for conversion of electrical current to light.
A layer in a semiconductor device that has an energy band gap that is larger than the energy band gap of a nearby layer can serve as a cladding layer to confine electrical carriers in a region of a ZnO based semiconductor device to improve the efficiency of the device for conversion of electrical current to light.
A substrate in a semiconductor device that has either p-type conduction or n-type conduction can be used to form desirable electrodes on the substrate and to make desirable electrical contact to the device.
A layer or substrate in a semiconductor device that has surface or bulk damage or surface roughness can alter the spectral intensity versus wavelength of light passing through the region that contains either bulk damage or surface roughness.
A layer or substrate in a semiconductor device that has one or more phosphor coatings can alter the spectral intensity versus wavelength of light passing through the phosphor coating. The coating can be comprised of one, or more than one, coating selected from the list consisting of, but not limited to, a single layer coating of an area, a composite coating of several phosphors, and a multilayer of different phosphors.
ZnO can be used as an active layer material. An active layer in a semiconductor device that has energy band gap lower than ZnO can be used for obtaining emission of light having wavelength longer than that emitted from a ZnO active layer. An active layer in a semiconductor device that has energy band gap higher than that for ZnO can be used for obtaining light having wavelength shorter than that emitted from a ZnO active layer.
A layer comprised of a metal or metal alloy can be used as either as a reflector layer and/or as an electrical contact layer. If a metal or metal alloy layer is used for both a reflector layer and an electrical contact layer then herein it is called a metallic reflector-electrical contact layer.
A distributed Bragg reflector (DBR) is a region comprised of layers of quarter-wave epitaxial layers that alternate between high and low refractive index, e.g., ZnO and a BeZnO alloy. Such a Bragg reflector layer region can have high optical reflectivity for wavelengths close to the spectral wavelength for which it is designed. Design parameters include the number of layer pairs, index difference, and the choice of layer material and its thickness for purposes of matching a Bragg reflector surface to its adjoining medium.
A flip-chip structure is one that allows contact to be made to a semiconductor device such as an LED that allows for extraction of generated light through the substrate. A commonly used flip-chip structure is one whereby one electrical contact is made to the substrate with an electrode design on the substrate that facilitates light extraction through the substrate. For example, a common electrode design is a perimeter strip that allows light to be transmitted through the interior region. The other electrical connection would be made to the topmost formed layer of the device, to an electrode with sufficient area and positioned in make electrical contact to a soft metal bump, such as indium (In). Contact to the metal bump can facilitate extraction of heat from the device during operation. Alternative designs are sometimes used. For example, an electrode in the shape of a square or circle that is located in the middle of the substrate face and occupies a small portion of the substrate face area is also commonly used.
Modification of the angle between the substrate edge and substrate face for a flip-chip can be employed an increase light extraction efficiency.
Such devices and capabilities include LEDs and LDs that emit in the UV and visible regions of the spectrum.
Semiconductor devices fabricated from ZnO based materials that can operate with increased performance, capability and function are desirable for use in many commercial and military sectors including, but not limited to devices and areas such as light emitters, photodetectors, FETs, PN diodes, PIN diodes, NPN transistors, PNP transistors, transparent transistors, circuit elements, communication networks, radar, sensors and medical image detectors.
Accordingly, it would be useful to provide ZnO based semiconductor films and structures and processes for preparing same that can be tailored to provide improved cladding and confinement layers, waveguide layers, carrier blocking layers, passivation layers, layers on which improved electrical contact can be made, and layers that increase quantum efficiency in the conversion of electrical energy to light energy for emitting devices, layers that are roughened or damaged for altering the spectral intensity versus wavelength for light, layers that contain one or more phosphors for altering the spectral intensity versus wavelength for light, layer structures that serve as mirrors, layers that are metallic reflector-electrical contact layers, layer structures that serve as reflectors, and layer structures that serve as waveguides.
It would also be desirable to provide, for example, a BeZnO alloy semiconductor layer tailored to have an energy band gap that is higher than that for ZnO for use as cladding layer and barrier layers for confinement of carriers a semiconductor device, as well as other types of layers and structures having selected material or functional characteristics for use in various semiconductor light emitting devices and methods.